1. Field of the Invention
The present invention relates to a rotary drive member such as a wheel, pulley, gear or the like that has a circumferentially extending drive formation with features of desired shape and size that are formed by the injection molding of thermoplastics material, but without a need for conventional measures to compensate for shrinkage of the thermoplastics material. More particularly, the present invention relates to a composite rotary drive member that has a circumferentially extending drive formation wherein peripheral portions of the drive member are formed by dual stage molding of thermoplastics material to first form a composite metal and plastic preform member that is permitted to undergo normal shrinkage before a relatively thin, band-like ring of thermoplastics material is molded in place about the periphery of the composite preform to define the drive formation with drive surface features of desired shape and size, with the band-like ring being so thin that it encurs a negligible amount of shrinkage.
2. Prior Art
Rotary drive members such as wheels, pulleys, gears and the like that are mountable on a shaft or other structure for rotation typically have been formed either from metal or from injection molded thermoplastics material. Forming a rotary drive member from metal has the advantage of providing a strong, rigid structure that can be configured to transfer relatively large torque, for example between a shaft on which the drive member is mounted and another drive member such as a gear or an end-less belt that engages the rotary drive member. Forming a rotary drive member from injection molded thermoplastics material has the advantage of providing a relatively lightweight structure that often can be formed at a cost that is relatively low, compared to the price of machining the drive member from metal.
It also is known to form one or more parts of a rotary drive member such as a pulley from injection molded plastics material, and to use the molded part or parts in combination with one or more separately fabricated metal parts to provide a rotary drive member that is formed as an assembly of plastic and metal components.
3. The Referenced Molded Pulley Patent
The invention of the referenced Molded Pulley Patent provides a rotary drive member, such as a wheel, gear, pulley or the like, that is formed by injection molding thermoplastics material to envelop selected portions of at least one preform insert. The injection molding of the thermoplastics material is a one-step process, and mold components are used that are sized to allow for shrinkage of the molded thermoplastics material.
In the preferred practice of the invention of the Molded Pulley Patent, two preform inserts made of metal such as steel are inserted into an injection mold to form a rotatable drive member that is a composite plastic and metal structure. One of the metal preform inserts is a tubular sleeve that defines portions of a hub of the rotatable drive member. The other of the metal preform inserts is an annular disc that defines portions of a radially extending web that serves to rigidly connect hub and rim portions of the rotatable drive member. The injection molded plastics material envelopes peripheral portions of the tubular sleeve as well as inner and outer edge portions of the annular disc, and defines a drive formation on the rim of the drive member for engaging another drive member such as a gear or an endless belt.
A problem not addressed by the referenced Molded Pulley Patent is that of providing a system for molding composite metal and plastic wheels, gears, pulleys and the like without having to take conventional steps (such as the over-sizing of mold components) to compensate for shrinkage of the molded thermoplastics material. Because shrinkage is encurred in the injection molding of thermoplastics materials, with the extent of the shrinkage typically being within the range of about 0.001 to about 0.015 inches per inch, and with shrinkage tending to be greater as the thickness of the molded part is increased, the presence of shrinkage on a pulley of 8 inches in diameter that is molded from thermoplastics material can cause the pulley to diminish in size from its original molded configuration by typically about 0.150 inch unless care is taken to properly oversize the mold adequately to compensate for shrinkage. Moreover, in determining the extent to which mold portions must be oversized in order to properly allow for shrinkage, a mold designer must take into account a host of factors that influence the way in which a molded part of particular cross section will cool. For example, the presence of hot spots in molded thermoplastics material that require prolonged cooling time will significantly increase the shrinkage that will occur.
Because the problems that are encurred in providing molds that will properly and reliably allow for shrinkage can add quite significantly to the cost of producing molded thermoplastic components, those who are skilled in the art long have sought a molding technique that will permit parts such as wheels, gears, pulleys and the like to be molded without a need to take shrinkage factors into account, while still providing a capability to accurately form drive surface features of desired size and shape.